Digital audio transmission for use in studio, stage or field applications

ABSTRACT

In studio, stage or field applications, high fidelity audio signals are transmitted to a remote processor in digital form in order to solve the problems of audio degradation, cross talk, ground loops and multi-cable problems associated with the analog transmission of multiple channels of audio over long distances. In one embodiment a TDM/FDM multiplexing system is utilized with increased bandwidth and dynamic range compared to data and telephone multiplexing systems to accommodate high fidelity requirements. In an embodiment involving a distributed system, multiple MUX and DEMUX modules are coupled in a distributive fashion along a light-weight transmission line, in which each of the modules is assigned a predetermined transmission frequency and with each of the modules having a number of audio inputs which are time-multiplexed for that particular MUX module and frequency. The Subject System precludes the necessity of running multiple audio cables to remote destinations, while at the same time providing an exceptionally quiet system, since the digital data stream is extremely tolerant to cross talk, ground loops, noise, signal attenuation, and non-linearity associated with conventional analog audio transmission.

FIELD OF INVENTION

This invention relates to the management of audio signals in stage,field, and studio environments and more particularly to a system for theaccommodation of a large number of audio signals with a minimum amountof physical cabling and signal degradation.

BACKGROUND OF THE INVENTION

High audio quality either in a studio environment or in a liveperformance environment is difficult to achieve especially where audioprocessing devices are chained together or indeed where all cabling isrun to a "patch bay" or to individual pieces of equipment. In order tofacilitate the recording or performance of music or vocal works, orindeed anything in which audio quality is important, in the past it hasbeen the practice to run multiple audio cables from various audiosources directly to a patch bay. The signals from the patch bay can berouted to audio processing equipment which may include simple mixingcircuits, distribution networks or equipment which introduces specialeffects. Maintaining fidelity in such systems is dependant in largepart, on the type of cabling utilized and the proper installation of allequipment and cables according to well-established audio engineeringpractice.

In general, so-called analog snakes provide the required signaltransmission. The audio snake is a heavy cable containing multipleshielded conductors and some unshielded lines, with the audio snakeweighing as much as 150 pounds for a large installation. Moreover, dueto the multi-conductor internal audio cabling, sophisticated largemulti-pin connectors are used at either end of the snake.

In general, the problem with providing a link between multiple audiosources and a remotely located processor starts with the problem offield use of audio equipment and analog transmission of signalstherebetween. For performances such as concerts, field remote taping andstudio work, these situations involve as many as forty or fifty audiosources in the recording or performance of the audio work. Typically inthese situations there are large numbers of sources of audio signals andother types of signals that are required to be transmitted from acentral point to distributive points, from distributive points to acentral point, or from one set of distributive points to another set ofdistributive points.

With all of the audio signals and control signals to be transmitted,large numbers of cables, each carrying an analog signal, have beenutilized in a brute force approach, to transmit signals betweendifferent sites and a centralized processing unit within a controlbooth, for instance. The problem with such a system is that it isexpensive, and suffers from the large number of cables which are routedin a piecemeal fashion across the floor.

More importantly, these signals typically have to travel long distanceswhich results in a substantial amount of signal degradation. It is alsovery inconvenient to lay down large numbers of cables quickly and thisis oftentimes a requirement in field usage. Additionally, with theadvent of the above-mentioned audio snake, the cabling is not onlyexpensive and heavy, but when numbers of side-by-side cables are encasedwithin a single sheath, cross talk is prevalent, along with the pick upof AC hum. Further, other signals than audio signals are frequentlyrouted through the audio snake such as control signals for the controlof lights, lasers or other special effects devices. Moreover, returnaudio is routinely routed through the snake back to monitors at thestage or to speakers on the stage. It is therefore difficult to transmitundegraded signals having a classical bandwidth of 20 Hz to 20 kHz overlong distances and with acceptable purity.

While the audio snake has in part taken the place of the routing ofmultiple wires from multiple audio sources directly to the patch bay,audio snakes may be 1-2" in diameter, and may carry as many as 50 audiocables side by side. As noted, each snake is expensive, as are therelatively complex connectors which are required at either end of theaudio snake to attach the cable to connector boxes.

Frequently what is found in some installations is that low level signalsand the higher level signals are mixed in the same snake cable whichprovides an opportunity for cross talk between the two sets of signals.If one has signals from microphones mixed in with signals for instancefrom power amplifiers, one can obtain regenerative feedback oscillationbecause one can have outputs being cross talked into the inputs and thiscan cause a continuous recirculation of the signal which leads tooscillation and other instabilities. Crosstalk as used herein refers tothe mixing of a signal in one channel into a signal transmitted inanother channel, with this problem being especially acute in studioswhere high quality is absolutely mandatory and channel separation is oneof the requisites.

It will be appreciated that the signal strength through each of thechannels of the audio snake can vary anywhere from a very low levelmicrophone output which is not recommended but is often done, to highlevels associated with speaker outputs. The mixture of different levelscan be troublesome even for line output levels on the order of 1 voltRMS which is usually available after a microphone output has beenamplified.

With respect to line loss, it has been recommended that for 250 ohmmicrophones the frequency losses above 10 kHz are significant if thecable length is on the order of 200 feet. The 10 kHz cutoff issignificantly in the audible range, and cable lengths of over 200 feetare not uncommon, especially for performances on stages and inauditoriums or athletic fields. This causes significant high end signaldegradation.

Degradation in general takes three forms. It can be the addition ofnoise, the introduction of non-linearities, or it can be the droppage ofthe higher frequency components of an amplified signal. Additionalnoise, including cross talk and ground loops, can occur at allfrequencies but is usually in the form of either 60 Hz hum or highfrequency interference, such as RF interference or spuriousoscillations.

In order to minimize cross talk, traditional analog cables have utilizedshielding in order to isolate the signals carried by the cables withinthe bundle. In less expensive audio snakes there is only one mainshield. This causes considerable cross talk problems because there isconsiderable unwanted interference between signals transmitted along theunshielded cables which are bundled together.

The ground loop problem is also a large problem in the field and even instudio applications where very low level signals have hum introducedbecause of improper grounding problems. This can be even more magnifiedwhen there is no DC isolation between the signals.

In order to minimize audio degradation in a large concert set up,instead of plugging a microphone directly into the audio snake, one canpre-amplify the microphone signal to boost the signal so that it can besent further distances. However, the amplification in and of itselfcauses problems. These problems include the aforementioned noise, andthe problems of ground loops, with the ground loop problem being severewhen amplifiers are improperly grounded, which is frequently the case innon-permanent, and some permanent set-ups. The ground loop problems canbe solved by the utilization of isolation transformers at either end ofcable. However, the utilization of isolation transformers in itselfintroduces problems of non-linearity and saturation.

Thus, in the past in order to solve the problems associated withmultiple audio signals in either stage or studio applications,relatively creative analog approaches have been utilized in order toprovide signal transmission from the audio source to a remote audioprocessing unit. However creative, all analog approaches suffer from theabove-mentioned problems.

SUMMARY OF THE INVENTION

In contradistinction to the utilization of conventional analog typetransmission between remotely located audio sources and processingcenters, in the Subject Invention the audio is converted to a digitalrepresentation at the audio source and then is transmitted in amultiplexed manner over a light-weight coaxial cable, or even afiberoptic or twisted-pair cable, with the multiplexing anddemultiplexing being of such a nature that little signal degradationoccurs. Also RF transmission of digitized audio is within the scope ofthis invention.

The advantage of digital transmission is that signal degradation is nolonger a significant problem. Thus ground loop problems are eliminated,as are 60 cycle hum and RF interference. The Subject System thereforeprovides an extremely "quiet" method and apparatus for transmitting highfidelity audio signals from as many as several hundred audio sources toremote processing units. This system also solves the problem of massiveamounts of expensive cabling in an audio installation which can getunmanageable; as well as solving the problem of the utilization of asingle but bulky and expensive audio snake with its attendant crosstalk, ground loop and other degradation problems.

In one embodiment of the Subject System, an FDM/TDM multiplexing systemis used. Synchronization of the TDM occurs by the provision of syncpulses for every strobe cycle rather than having an additional linecarrying a synchronization pulse. The sync pulse in one embodiment, isdifferentiated from data by its signature so that it will never appearto the demultiplexing circuits as legitimate audio data. In oneembodiment, four audio signals are digitized into a single serial datastream. The stream of data has a rate of approximately 4 megabits persecond which in turn is frequency division multiplexed, such that foreach set of four audio sources which are time division multiplexed thereis a predetermined frequency associated therewith. These frequencies canbe chosen either for coaxial cable or fiberoptic cable, with coaxialcable being able to handle several hundred megabits per second orseveral hundred megahertz of bandwidth. Since each MUX channel consumesclose to 6 MHz and since there are typically 4 audio channels per MUXmodule, one can multiplex several hundred audio signals into a singlecoaxial cable. Six megahertz wide frequency channels are selected as anoptimum trade-off of channel space overhead vs hardware efficiences.

With respect to the multiplexing system, the individual microphone oraudio source outputs can be fed through individual cables to acentralized multiplexer from whence the coaxial cable is lead to aremote demultiplexing unit. While this system solves the problem of longlengths of cable, it is not as convenient as utilizing a distributedsystem in which MUX modules are utilized in close proximity to groups ofaudio sources. In a preferred embodiment, the connecting cable spline isfed to modules located remotely; and in a further embodiment, instead ofcutting a wire and inserting a "tee" connector, each MUX module may beconnected to the next adjacent MUX module through suitable input andoutput connectors by daisy chaining MUX and DEMUX units. Note that DEMUXunits may be used at the stage to demultiplex return audio or othersignals.

One advantage of employing digital audio transmission by this method inthe recording or processing of audio for stage and studio performancesis a lower cost for longer distance transmissions, as opposed to thetraditional audio snake which cable may be on the order of severaldollars per foot in cost. The Subject System is implemented at a fixedcost, plus a substantially lower incremental cost for longer distances.

Moreover, since every DEMUX unit is exposed to all multiplexed signalsit becomes possible to reallocate a DEMUX channel in real time from evena remote location. Thus, the setup configuration on stage can be alteredor modified at a control booth, with different microphones beingreassigned to different channels. This prevents having to physicallymove or rewire microphones which are already in place on a stage. Thisadaptability can be easily accomplished by having every MUX and DEMUXmodule continually "listen" on DC signalling or a pre-assignedconfiguration channel.

Most importantly, the system is a "quiet" system in which there isnegligible signal degradation in the transmission. One can transmit allthe required signals several thousand feet and obtain the same amount ofquality as if one were transmitting the signal only several inches; thisis because digital signals are less susceptible to degradation.

The utilization of digital communication solves the problem of groundloops because it is very tolerant of analog interference. The system isessentially isolated so that if one has a ground loop on the coaxialcable it does not manifest itself in the audio signal.

The Subject System also eliminates the problems of expensive multi-pinconnectors, cables, and connector boxes and solves the problem of theweight of the audio snake cable in which one does not have to worryabout supporting a very heavy cable which can be a problem in a largeinstallation where these cables can weigh several hundred pounds. Tryingto suspend an audio snake from an overhead is eliminated by the use oflight weight coaxial or fiberoptic cables involved in the SubjectInvention. RF transmission provides even further convenience. Also itwill be appreciated that after digital encoding there is noamplification and therefore the problems associated with non-linearamplification, oscillations and other degradation is eliminated by thedigital transmission between the audio source and the audio processingunit. Additionally, the digitally coded signal is easily accepted bydigital processing equipment that is becoming increasingly prevalent inthe audio industry. This multiplexing and transmission scheme is ideallysuited for digital audio processing equipment which frequently convertsdigital signals into analog signals for transmission even when thefollowing stage is another digital processor. By using the SubjectInvention's digital transmission method, needless conversions and theirassociated cost, complexity and degradation, are eliminated. Finally,the transmission network that is established by the system is notinherently limited to the distribution of audio signals. Other data thatcan be adapted and distributed by this system includes, lighting, timecodes and synchronization, video, MIDI, and other forms of data.

PRIOR MULTIPLEXING SYSTEMS

By way of further background, in prior multiplexing systems frequencydomain multiplexing systems have been used primarily for datacommunications, as have time division multiplexing systems. For datacommunications, especially with respect to the Wangnet system, themultiplexing and the demultiplexing will not support data transferfaster than a 19,000 bit per second rate per channel. While this issufficient for digital data, a typical high fidelity audio signal whenPCM coded is close to a megahertz in bandwidth. Thus, typical datacommunications multiplexing and demultiplexing is not capable ofsupporting high fidelity audio.

For audio bandwidths on the order of 1 megabit per second per audiochannel, and with the possibility of several hundred audio channels, thesystem must accommodate several hundred megabits per second in realtime. Typically there may be as many as 24-48 audio channels in a liveperformance. In such a case, in order to provide the subject digitaltransmission the system would be required to accommodate 48 megabits persecond in real time.

As mentioned before, in the data communication field there are nomultiplexing and demultiplexing systems which can handle thisrequirement and still be a distributed system, where audio signals maybe introduced or extracted from anywhere along the transmission media.There are however multiplexing and demultiplexing systems in thetelephone field. However transmission systems in telephony are orientedto different criteria and are not acceptable for high fidelitytransmission. Transmission over telephone lines is relativelyband-limited in that for telephone quality signals, all that is requiredis a 300 Hz-3 kHz passband. Anything beyond 3 kHz is cut off. Thereforethose multiplexing and demultiplexing systems utilized in the telephonefield cannot support the requirement for high fidelity.

Moreover, the dynamic range supported by telephone multiplexing systemsis considerably less than the dynamic range required for the studio orlive performance situation. For instance telephone PCM information isrepresented by an 8 bit number, where the dynamic range is 1 part in 256or 48 dB. In the high fidelity audio situation, a minimum 16 bit numberis required. Therefore, the telephone will not accommodate the dynamicrange of 1 part in 65,536 or 96 dB. This means that the telephone systemmultiplexing and demultiplexing systems cannot even come close to thecapability that is demanded in a high fidelity system. Again, thetelephone multiplexing schemes are not designed to be distributed, butare point-to-point.

Finally, it should be noted that there are television multiplexing anddemultiplexing systems for the distribution of TV and digital audiosignals such as described in U.S. Pat. No. 4,513,315 and U.S. Pat. No.4,704,715. These systems were not envisioned as being usable ordesirable in a studio or live performance situation; nor were theydesigned for studio quality audio processing. Moreover, cable andsubscriber TV multiplexing requirements are different from thoseassociated with studio quality audio. In a CATV system the audio signalsoriginate from one point and thus the CATV system is not a trulydistributed system. Additionally, these CATV systems go to greatlengths, in terms of added complexity and channel inefficiences, tominimize problems that are associated with wide-area CATV distributionnetworks that would not be encountered in the audio transmissionapplications envisioned here. Specifically, the CATV systems employcomplex modulators to address the problems of signal echo encountered inless than ideal outdoor installations.

Other patents illustrating TDM/FDM systems are U.S. Pat. Nos. 4,715,029;4,598,398; 4,589,018; 4,590,595; 4,510,598; 4,553,101; 4,479,240;4,438,511; 4,397,019; 4,389,538; 4,312,062; 4,199,660; 4,171,467;4,075,429; 4,013,842; 3,959,595; 3,736,374; 3,655,917; 3,637,940;3,573,379; 3,519,747; 3,471,646; 3,435,147; and, 3,370,128.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the Subject Invention will be betterunderstood in connection with the Detailed Description taken inconjunction with the Drawings of which:

FIG. 1 is a diagrammatic representation of prior art system involving aconcert stage in which multiple audio sources are connected from thestage to a control booth through an analog audio transmission system,usually employing the aforementioned audio snake;

FIG. 2 is a schematic diagram of the prior art system of FIG. 1illustrating the multiple audio sources and, more importantly, theutilization of multiple shielded audio cables within the snake both forthe transmission of signals from the stage to the control booth and forthe transmission of return audio and control signals to the stage;

FIG. 3 is a diagrammatic illustration of the Subject System in whichaudio transmission to and from the stage is in the form of multiplexeddigital audio, thereby to provide a quiet, attenuationless transmissionsystem for high quality audio;

FIG. 4 is a diagrammatic illustration of a distributed implementation ofthe system of FIG. 3 in which MUX and DEMUX modules are daisy chainedalong the transmission line across the stage to provide for efficientand easy accommodation of multiple sources and destinations for theaudio signals;

FIG. 5 is a block diagram of the Subject Invention illustrating the useof one-way digital multiplexed transmission between multiple analogaudio sources at one location and multiple destinations for analog audiochannels at a second location;

FIG. 6 is a block diagram illustrating two-way communication via digitalmultiplex transmission between a studio/stage location and a booth;

FIG. 7 is a diagrammatic illustration of a distributed system in whichconnection of MUX and DEMUX units to a transmission line is accomplishedthrough the utilization of a "tee", with MUX and DEMUX units tappedalong the line as required;

FIG. 8 is a diagrammatic illustration of a distributed system in whichMUX or DEMUX modules are daisy chained along a transmission line; and,

FIGS. 9A and 9B are schematic and diagrammatic illustrations of oneembodiment of a multiplexing system for use in the Subject Invention, inwhich TDM/FDM multiplexing is illustrated.

DETAILED DESCRIPTION

Referring now to FIG. 1, in one of prior art installation multiple audiosources in the form of microphones 10 are positioned across a stage 12and are connected via numerous individual cables 14 to a central patchbay 16 at one side of stage 12. In addition to the connection of theaudio sources via cables 14 to the patch bay, patch bay 16 also includeslines 18 to apply return audio to monitors or speakers 20 located at orin the vicinity of the stage.

The audio cables, corresponding one each to an audio channel, are routedthrough the aforementioned audio snake 22 to a control booth 24 locatedat the rear of the auditorium at a distance, usually in excess of 200feet from the stage. It will be appreciated even in modest theaters suchas illustrated by the seating arrangement 26 that significant signaldegradation occurs due to the length of audio cable that is utilized,and due the aforementioned cross talk and ground loop problems. Also, asmentioned hereinbefore, the length of the audio snake requires aconsiderable amount of weight be suspended when running the snake fromthe stage to the control booth. Moreover, personnel running the controlbooth are often faced not only with low level signals and noise or humpicked up from the long run of cable, they are also confronted with theinitial set up of the stage in terms of placing the microphones and therunning of the large numbers of cables to the patch bay. In a typicalinitial set up it may take two hours or more to position cables andmicrophones across the stage so that the cables will be neat and orderlyand not be in the way, and so that cross talk, noise and ground loops inthe set-up are minimized. Additionally, modifying the set up isdifficult in the system depicted in FIG. 1 because of the tangle ofcables, the complexity of cable markings, and the inability to identifythe particular cable associated with a particular input channel.

It will be appreciated that the entire system illustrated in FIG. 1involves analog transmission techniques, including analog connection andswitching, in order to be able to effectuate the audio transmission fromthe stage to the control booth and visa versa. In order to switch, forinstance, a microphone from one channel to another channel it isnecessary to physically reconnect this particular microphone at thepatch bay, a task which is both cumbersome and time consuming.

Referring to FIG. 2, not only are the problems in the prior art systemFIG. 1 limited to the stage. Rather, as can be seen in this diagrammaticillustration in the more expensive of the audio snakes each individualaudio line is a shielded cable here illustrated at 30 which is runusually in parallel with as many as 49 other audio cables going in thedirection of control booth 24. Likewise there may be 2 or more audioreturn lines 32 again running in parallel but in opposite directions tothe aforementioned audio lines. Should special effects be desired onstage, control signals may be delivered over lines 34 to patch bay 16where they are distributed to the various devices which they control.The incoming audio is processed at a processing unit 36, whereas thereturn audio signals and control signals may be generated at a separateunit, here illustrated by reference character 38. It will be appreciatedthat the connection of audio snake 22 to processor 36 and processor 38may be done through a patch bay junction box 40.

It will be appreciated that the processing which takes place at thecontrol booth usually involves a mixer, the outputs of which may then becoupled to graphic equalizers, compressors, expanders, special effectsunits for introducing time delays, enchancers of every kind, and storagedevices such as magnetic storage, either tape or disc.

With respect to the ancillary control and return audio processing unit,this unit may in fact be coupled to processing unit 36, or the mixerthereof, to provide return audio signals for the on-stage monitors. Itwill be appreciated that the audio signals from patch bay 16 may havelevels on the order of 100 mV RMS, whereas return audio may includeamplified return audio signals which may be an order of magnitudehigher.

In general, the difficulty of long distance transmission of audiosignals in an analog fashion is both the likelihood of cross talk, evenin the best of shielded cable, as well as the pick up of AC hum from 60cycle sources both from lighting and from ground loops and otherequipment on stage. Additionally, if any motorized devices are utilizedwithin the area such as lighting equipment motors or on stage motordrives, the effect of actuating these devices is extremely deleteriousto the recording of quality audio or even the reproduction of audio foraudience usage.

Of course any RF interference from radio sources which are quitefrequently in the vicinity of the stage produce unwanted transienteffects.

The ubiquitous ground loop problem is ever present. As discussed, in apoor set-up the level of the AC hum can completely overwhelm the audiosignals. AC hum is introduced primarily because of grounding andisolation problems which are particularly severe in view of the mobileand distributed utilization of such systems due to improperconfiguration of faulty connectors or faulty grounding of any kind. Suchnoise can also occur from improper set up due to poor adherence tocommonly accepted practices for connection and routing of audio cables.

Referring now the FIG. 3, in the Subject Invention stage 12 is providedwith the self-same microphones or audio sources 10 which are againrouted to a central location here illustrated by reference character 40.This central location is provided with a multiplexing/demultiplexingunit 42 which, in a preferred embodiment, both time and frequencymultiplexes the incoming analog signals by first converting them todigital form through analog-to-digital converters. This provides serialtransmission of the digitized audio over a multiplexed digital audiolink 42 to a remote location, in this embodiment control booth 24. Whilein the illustrated embodiment of FIG. 3 there still exists multipleshort analog cables, the majority of the transmission path isaccomplished by a coaxial cable, a fiber optic cable, or indeed an RFlink should such be desired. This link between stage and control boothis extremely light weight and has the necessary bandwidth to accommodatethe transmission of the digitized audio to and from the remote location.As will be appreciated coaxial cable has a bandwidth in excess of 500megahertz, clearly sufficient for multiple channel audio transmission.Of course fiber optic cable has a bandwidth almost two orders ofmagnitude greater than coaxial cable. However, fiber optic cable, may beutilized if desired.

For runs of any length, especially in excess of 100 meters, the SubjectSystem is exceptionally quiet because of the utilization of digitalaudio. The digital format does not suffer from degradation problems inthe way that analog signalling does; and all of the aforementionedground loop and cross talk problems are completely eliminated due to thetransmission of digitized audio in whatever multiplexed format. Thistransmission scheme is as near to a lossless system as can be achieved.Signal quality problems are insignificant compared to those associatedwith analog audio transmission. Sufficient bandwidth is currentlyavailable through the utilization of TDM/FDM systems when adapted forhigh fidelity audio transmission through the utilization ofanalog-to-digital converters and digital-to-analog converters having 16to 18 bit capacities. Thus while frequency and dynamic range problemsexist with respect to the multiplexing systems utilized in telephonesystems, the advances in analog-to-digital conversion anddigital-to-analog conversion make the fabrication of a high fidelitymultiplexing system within the state of the art. The FDM/TDM systemsuggested in one embodiment of the Subject Invention is at least inorder of magnitude faster than the systems provided for datacommunications. State of the art technologies capable of handlingdynamic ranges of at least 90 dB are presently available. Thus dynamicrange problems associated with prior telephone multiplexing do not existhere.

Also the problem of the high frequency cut off for analog snakes above10 kilohertz is completely eliminated through the use of digitizedaudio; and unlike analog signalling, the transmitted audio signals donot degrade as distance increases.

Referring now to FIG. 4, a distributed embodiment of the Subject Systemeliminates the tangle of short audio lines between the audio sources andthe point at which the audio sources are multiplexed for transmission toa remote location. In this figure the audio sources, here alsoillustrated as microphones 10, are placed where ever desired acrossstage 12. In this embodiment 4 microphones are associated with a singleMUX unit 50, with MUX units 50 being distributed along cable 42 in adaisy chain fashion. It will be noted that cable 42 is terminated at 52,as would be expected. In addition to MUX units 50 being distributedalong cable 42, DEMUX units 54 for processing signals from control booth24 are also distributed along cable 42. In the illustrated embodimentthe DEMUX units are utilized to provide signals to monitors andloudspeakers 56 located adjacent stage 12. It will be appreciated thatthe utilization of a distributed system at the stage provides not onlyfor shorter analog cables to be utilized between the sound source andthe distributed point, it also eliminates clutter on stage and moreeasily permits definition or identification of the signal source withouttagging a tangle of microphone cables. A single coaxial or fiber opticcable may be snaked across the stage from which ultra short cablingruns. Thus this distributed system is not anywhere near as obtrusive asthe situation depicted in either FIG. 1 or FIG. 3.

In both the FIG. 3 and FIG. 4 embodiments there is in fact two-waycommunication permitted between locations so that, for instance, in FIG.3, a demultiplexing circuit in unit 42 can be provided to demultiplexreturn audio or control signals for monitors 56.

While the aforementioned TDM/FDM multiplexing system is useful in theSubject System, virtually any type of transmission system which providesa serial stream of data to and from the spaced apart locations is withinthe scope of this invention. Token-ring, packet switching, or othernetworking technologies are also encompassed within the scope of thisinvention, when used for the real-time transmission of audio data.

Referring now to FIG. 5, in essence the Subject System involves multipleanalog audio sources 60 which have outputs provided to a digital encoder62 which encodes the signals in such a manner that they may betransmitted as a digital stream on transmission line or path 64 to aremote location at which point digital decoding 66 is accomplished. Thedigital decoding reconstructs the original audio channels and routesthese channels to multiple destinations 68. What is thus depicted inFIG. 5 is a simple one-way digital multiplexed audio transmission systemwhich eliminates all of the aforementioned problems with analoglong-distance signalling.

Referring to FIG. 6 should two-way communication be desired a digitalencoder 70 may be provided with signals from multiple sources 72, whichsources may include both audio and control signals. The output ofdigital encoder 70 is coupled to transmission line 64, with thesesignals being decoded by a digital decoding unit 74, from whence thereturn audio and control signals are reconstructed and passed to asuitable processing unit 76.

In summary, the Subject System is one in which multiple audio channelsare transmitted to a remote location by use of digital multiplexedtransmission techniques.

Referring now to FIG. 7 what is depicted here is a distributed networkin which MUX units 50 are distributed along cable 42 through theutilization of "tee" type connectors here illustrated at 80. Alsodistributed along cable 42 are DEMUX units 54' and 54", with a DEMUXunit 54' demultiplexing signals directed to on stage monitors, whereasDEMUX unit 54' demultiplexes signals provided to a MIDI unit 82 or anyother controller. Here MIDI is a common designator for musicalinstrument digital interface, which units are available commercially.MIDI units are conventionally utilized for the transmission of digitaldata between musical instruments and are not utilized for the real-timetransmission of audio information.

Referring now to FIG. 8, a distributed system is illustrated in whichDEMUX units 50 are daisy chained along transmission line 42 for thepurposes described above. The advantage of the utilization of such adaisy chaining system is the elimination of "tee" type connections,although suitable other types of connections are necessary for each MUXor DEMUX unit distributed along line 42.

MULTIPLEXING SYSTEM

Referring now to FIGS. 9A and 9B, a TDM/FDM system is illustrated inwhich for each MUX module 50 there are provided four inputs, hereillustrated by sources A, B, C, and D. The outputs of these sources areamplified at 90, 92, 94, and 96 by conventional amplification means andare provided to specialized high speed 16 bit analog-to-digitalconverters 98, 100, 102 and 104, available from several commercialsources. It is the property of these analog-to-digital converters thatthey have sufficient dynamic range to accommodate the types of acousticsignals applied thereto. The dynamic range in general for the highfidelity recording and reproduction systems for which the system is tobe used should be in excess of 90 dB and can require that theanalog-to-digital converters have a 16 bit resolution; although 18 andgreater bit capacities are currently within the state of the art. Theseanalog-to-digital converters are strobed at a frequency of 48,000 timesa second to provide for the time multiplexing which is accomplishedconventionally by time multiplexing unit 110. In one embodiment theoutput of each individual analog-to-digital converter is sampled by theTDM unit, which after sampling one analog-to-digital converter proceedson to the next. The output of the time division multiplex unit is a bitstream which is coded in a conventional manner to identify not only theinformation from each of the analog-to-digital converters but also theidentity of the analog-to-digital converter. This bit stream ismodulated at modulator 112 by a frequency f_(x), which in one embodimentis a frequency between 10 and 500 megahertz so as to be demodulatable assuch at the remote location. This provides a signal S₁ to a summing node114, with signals S₂ and S₃ . . . S_(n) applied to summing nodes 114.Thus the signals from each of the MUX modules are applied to cable 42.

The signal on cable 42 is sampled through splitter 118 at a remotelocation having a DEMUX module here illustrated by 120 to include afrequency demodulator 122 which demodulates a predetermined frequency off_(x) at any given time. The output of demodulator 122 therefore selectsthe particular MUX module to be received, at which point data therefromis passed to a time division demultiplexer 124, the outputs of whichcorresponds to channels A, B, C, and D. The outputs of the time divisionmultiplex unit are digital signals which are then supplied todigital-to-analog convertors 126, 128, 130, and 132 which reconstructthe original audio signal on the respective channel. The output ofdigital-to-analog convertors 132 are conventionally amplified at 134,136, 138, and 140 and are thereafter supplied to an audio processingunit 150 which processes the analog audio signals as desired. Both A/Dand D/A converters are not necessary if the source or destinationequipment is capable of supplying or using digital audio signalsdirectly.

Thus, the subject TDM/FDM system is enhanced over the aforementionedWangnet system in that the data capacity is significantly greater thanWangnet capability and on the order of 4 megabits/second per MUXchannel. This is 3 or 4 orders of magnitude greater than the ratesutilized for serial data communications multiplexing systems.

With respect to telephone type TDM/FDM systems, the dynamic rangeaccommodatable by the Subject System is two orders of magnitude largerthan that associated with telephone systems. Moreover, telephonemultiplexing systems do not typically employ a TDM/FDM combination.Moreover, channel spacing in the Subject System vis-a-vis that of atelephone system is again three orders of magnitude larger.

It will be appreciated that channel spacing is important in the SubjectSystem in order to provide for required audio fidelity through higherdata rates.

It will also be appreciated that the Subject TDM/FDM System is desirablyoperated in the VHF frequency band to facilitate the use of coaxialcable. However, the Subject System is different from the above mentionedCATV systems in that the systems of the aforementioned patents do notenvision multiple audio sources for distribution of these audio sourcesalong a transmission line. Nor do these patents contemplate processingthe audio sources in multiple separate channels at any destination. Notethat in U.S. Pat. No. 4,513,315 the TDM/FDM conversion is to reducesignal echoes. Moreover, the Subject System employs large numbers ofaudio channels multiplexed both in time and frequency domains, withfrequency allocation for MUX designation, not type of signal.

With respect to U.S. Pat. No. 4,704,715 this is in essence apoint-to-point cable system in which optical fibers are utilized and inwhich there is frequency division multiplexing only in terms of phaseshifting for optical purposes. This system is not a distributed system.Nor does it contemplate the accommodation of multiple audio sources.

It will be appreciated that the Subject System contemplates multipleaudio source in terms of more than just left and right stereoprocessing. The term multiple sources, as used herein, means sourcesthat are distinct in location in terms of the production of the sound,or distinct in terms of the audio that is produced. What this says isthat the multiple source nomenclature used herein does not contemplatestereo processing a single sound source which develops left and rightcomponents; but rather sources which are physically distributed orsources which have no acoustic relation, one to the other.

Having above indicated a preferred embodiment of the present invention,it will occur to those skilled in the art that modifications andalternatives can be practiced within the spirit of the invention. It isaccordingly intended to define the scope of the invention only asindicated in the following claims:

I claim:
 1. A quiet high fidelity audio transmission system for use instage, field, and studio applications for long distance transmission ofmultiple channels of high fidelity audio between spaced apart locationswith reduced degradation and noise, in which multiple sources of audiosignals are available from different points at one location, comprisinga digital snake including:means for multiplexing said multiple sourcesof audio signals at said one location so as to provide a digital datastream with a digital format corresponding to separate audio channels; atransmission line having a suitably wide bandwidth exceeding 1MHz/channel for transmitting said digital data stream from said onelocation to a remote location; and, means for demultiplexing thetransmitted signals at said remote location so as to produce analogaudio signals in multiple high fidelity audio channels, whereby problemsassociated with analog transmission of audio signals over long distancesare eliminated by using the multiplexed digital transmission format,said system being a distributed system including a number of MUX unitstapped along said transmission line, thereby to minimize analog cablingrequirements and complexity at said one location.
 2. The system of claim1 wherein said multiplexing and demultiplexing means includes TDM/FDMmultiplexing means.
 3. The system of claim 1 wherein said multiplexingmeans including analog-to-digital converters.
 4. The system of claim 3wherein said analog-to-digital converters are at least 16 bit devicesfor adequate hi-fi dynamic range.
 5. The system of claim 1 wherein saiddemultiplexing means includes digital-to-analog converters.
 6. Thesystem of claim 5 wherein said digital-to-analog converters are at least16 bit devices for adequate hi-fi dynamic range.
 7. A digital snake forthe long distance transmission of multiple channels of audio fromseparate audio sources at locations distributed along a transmissionline, comprising:means at each location for multiplexing the output ofmultiple sources of audio into a digital data stream; means for applyingsaid digital data stream to said transmission line, said transmissionline having a bandwidth of at least 1 MHz/channel; and, means coupled tosaid transmission line at a remote location for demultiplexing saiddigital stream into separate channels of audio.
 8. The system of claim 7and further including more than one source of audio at each multiplexinglocation.
 9. The system of claim 8 and further including means forapplying bidirectional digital signals on said transmission line andmeans for demultiplexing selected bidirectional digital signals atselected distributed locations on said line, whereby said systemaccommodates bidirectional long distance transmission of audio.
 10. Thesystem of claim 9 and further including means within said multiplexingmeans for generating digital control signals multiplexed within saiddigital data stream; and,means within said demultiplexing means fordemultiplexing said control signals, whereby control signals can betransmitted along with digitized audio signals.